Based on the principle of operation, electric generators are categorized into synchronous generators, induction generators, and direct current generators. In any of these generators, when a magnetic field is applied to the coil of wire, an electromotive force is generated in the coil.
As a small synchronous generator, a bicycle dynamo is generally well known. In the bicycle dynamo, when a permanent magnet with south poles and north poles alternately arranged is turned, iron pieces provided to the coil are magnetized and an electromotive force is generated in the coil.
Such iron pieces provided to the coil are close to the permanent magnet at a small distance so as to efficiently apply a magnetic field to the coil. If the permanent magnet with a strong magnetic force is used to apply a strong magnetic field to the coil, a strong attractive force of the permanent magnet to the iron pieces is generated between the iron pieces and the permanent magnet. The force that the attractive force exerts on a rotation axis is called cogging torque. If the cogging torque is strong, various problem such as running torque fluctuations, abnormal vibration, and noise arise. For example, in the case of a bicycle dynamo, a heavy load is applied to the wheel when the wheel is turned. In the case of a wind turbine generator or the like, if cogging torque is strong, starting torque of a rotor becomes strong. In addition, the resistance to continuous rotation of the rotor becomes high. For this reason, it is difficult to generate electricity in light wind conditions.